Two-stage, single magnet mass spectrometer

ABSTRACT

The spectrometer includes a single magnet having two opposing pole faces each spanning at least a 270* sector. A 270* sector nonmagnetic chamber is juxtaposed in the air gap of the magnet and is provided with partitions for forming 90* and 180* sector portions. An ion source generates a relatively wide, parallel beam of ions which is accelerated into the 90* sector portion. A common partition between the 90* and 180* sector portions and the partition at the exit end of the 180* sector portion are each provided with a narrow slit through which particular ions are focussed to obtain the mass separation function of the spectrometer.

United States Patent Davis Jan. 16, 1973 TWO-STAGE, SINGLE MAGNET MASS SPECTROMETER Inventor: William D. Davis, Schenectady,

NY. Assignee: General Electric Company Filed: Dec. 8, 1972 Appl. No.: 96,117

u.s. c1...2'6 4'i'.9i IE, 250/419 G,"250'/41.'9 s1 1m. c1 ..H0lj 39/34 [58] Field of Search....250/4l .9 G, 41.9 SB, 41.9 ME

[56] References Cited UNITED STATES PATENTS 2,486,199 10/1949 Nier ..250/4l.9 X 3,231,735 1/1966 Peters..... ....250/41.9 2,775,706 12/1956 Wiley ....250/4l.9 3,387,131 6/1968 Helmer ..250/4l.9

FOREIGN PATENTS OR APPLlCATlONS 141,559 12/1960 U.S.S.R ..250/4l.9

Primary Examiner-William F. Lindquist Attorney-Paul A. Frank, John F Ahcrn, Julius J. Zaskalicky, Louis A. Moucha, Frank L. Neuhuuser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT The spectrometer includes a single magnet having two opposing pole faces each spanning at least a 270 sector. A 270 sector nonmagnetic chamber is juxtaposed in the air gap of the magnet and is provided with partitions for forming 90 and 180 sector portions. An ion source generates a relatively wide, parallel beam of ions which is accelerated into the 90 sector portion. A common partition between the 90 and 180 sector portions and the partition at the exit end of the 180 sector portion are each provided with a narrow slit through which particular ions are focussed to obtain the mass separation function of the spectrometer.

1 Claim, 3 Drawing Figures PATENTEDJAH 16 1975 INVENTOR ILLIAM D. DA

TWO-STAGE, SINGLE MAGNET MASS SPECTROMETER My invention relates to a new mass spectrometer, and in particular, to a spectrometer using a single magnet and a chamber formed into 90 and 180 sector portions for providing two stages of mass separation.

The mass spectrometer has many uses such as in the technology of gas analysis for detecting trace amounts of a particular gas in a sample gaseous mixture, and in high vacuum technology as a helium leak detector. Although the single stage mass spectrometer is of value, the advantages of two or more stages of mass separation often outweigh the additional cost and device complexity of the multi-stage spectrometer. The chief advantage of a two-stage spectrometer is the reduction of the background intensity of unwanted ions detected at the exit slit of the device. At relatively low pressures in the order of l torr, these unwanted or background ions arrive at the exit slit primarily by being reflected or scattered from the walls of the spectrometer and may differ greatly in mass from that of the ions being focussed on the exit slit. At relatively high pressures in the order torr and higher, gas scattering also occurs causing an increase in the background intensity primarily near the ionization peak corresponding to the principal gas sample being ionized in the spectrometer. Two stages of mass separation thus act as a double filter and thereby usually reduce the background ions by several orders of magnitude.

The conventional structure of two-stage mass separation is a series arrangement of two separate 90 mass analyzers such that the exit slit of the first analyzer becomes the entrance slit of the second. A disadvantage of this conventional structure is the separation of the two magnets of the two spectrometers (analyzers) by twice the radius of curvature and thus problems of large size, alignment and synchronization of the two separated fields can occur.

Therefore a principal object of my invention is to provide a two-stage mass spectrometer which overcomes the large size, alignment and synchronization problems of two separate single stage spectrometers connected in series.

Another object of my invention is to provide a twostage mass spectrometer constructed with a single magnet.

A further object of my invention is to provide a new method for achieving two stages of mass separation when utilizing only a single magnet for generating the magnetic field in the spectrometer.

Briefly stated, my invention is a two-stage, single magnet mass spectrometer utilizing a single magnet having two opposing pole faces each spanning at least a 270 sector, and a 270 sector nonmagnetic chamber juxtaposed in the magnet air gap. The chamber is divided into 90 and 180 sector portions by means of a common partition provided with a first narrow slit through which the particular ions to be monitored are focussed for passage from the 90 sector portion to the 180 sector portion of the chamber. An ion source is positioned adjacent an entrance partition of the 90 sector portion and generates a wide, parallel ion beam which is accelerated into the 90 sector portion. The 180 sector portion is provided with an ion exit partition having a second narrow slit through which the particular monitored ions are again focussed for passage to an ion collection means. The and sector portions thus form two stages of a mass spectrometer and the use of a single magnet causes the magnetic fields in the two stages to automatically track each other.

The features of my invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the following drawings wherein like parts in each of the several figures are identified by the same reference character, and wherein:

H0. 3 is a perspective view of a two-stage, single magnet mass spectrometer constructed in accordance with my invention;

H0. 2 is an enlarged perspective view, partly broken away, of the 270 sector nonmagnetic chamber juxtaposed in the magnet air gap, and also illustrates a second embodiment of the magnet pole face; and

W6. 3 is a cross-sectional view of the chamber illustrated in H6. 2 in top view with the ion source chamber component partly broken away.

Referring now in particular to FIG. 1, there is shown a view of my mass spectrometer which comprises a single C-shaped magnet designated by numeral 10 and a nonmagnetic chamber ll juxtaposed in the air gap between the parallel pole faces l2, 13 of magnet 10. Chamber ll contains the ion source, magnetic lenses and ion collector components of the spectrometer. Although magnet it) may be of the permanent magnet type, it is more preferably an electromagnet energized by coils l4 and 15 which are wrapped around the leg of the magnet containing the air gap. The advantage of the electromagnet is that it permits control of the magnetic field across chamber lll. Alternatively, a single coil may be wrapped around the opposite leg of the magnet, if desired. Coils 14 and 15 are assumed to be identical coils and are shown connected to a suitable source of direct current (DC) power designated V Alternatively, coils 14 and 15 may be connected in series across a suitable DC power source. Magnet 10 may be of virtually any shape in cross section, a circular shape being indicated in FIG. 1. The pole faces 12, 13 of magnet 10 may also be circular in cross section as illustrated in FIG. 1, or may span a 270 sector as illustrated in FIG. 2, or span any angle between 270 and 360.

Nonmagnetic chamber 11 is coaxial with the leg of magnet 10 containing the air gap and is generally of cylindrical shape having a diameter somewhat greater, smaller or equal to the diameter of such magnet leg. Chamber 11 is totally enclosed (except for a gas inlet, exhaust and electrical conductor passages) and in some applications at least one of the top or bottom surfaces thereof (as depicted in the figures) may be formed by the adjacent magnet pole face. Chamber 11 is comprised of a 270 sector portion 20 containing the magnetic lenses component of my spectrometer, and a 90 sector portion 21 containing the ion source and ion collector components. The 90 sector portion 21 may be of equal height as the 270 sector portion as shown in FIG. 11 or of greater height to accommodate a larger ion source as illustrated in H6. 2. The details of the ion source magnetic lenses, and ion collecting components in chamber 11 will be described in detail with reference to FIGS. 2 and 3. It is suffice to state at this point that the ion source generates a relatively wide, parallel beam of ions that are accelerated into the mass separation portions (magnetic lenses) of chamber 11, and the ion collecting means is positioned at the exit slit thereof. Thus, the 90 sector nonmagnetic chamber 21 when combined with the 270 sector nonmagnetic chamber forms a cylindrical shape as depicted in FlG. l. The various electrical conductors and tubing utilized in my spectrometer are not illustrated in the FIG. 1 embodiment for purposes of simplicity but are illustrated in FIGS. 2 and 3.

Referring now to FIGS. 2 and 3, the details of the 90 sector chamber 21 and the 270 sector chamber 20 will be described. The ion source of my spectrometer includes a rectangular shaped chamber 22 suitably supported in chamber 2i, and having front and back walls parallel to the ion entrance partition 23 of chamber 20. Chamber 22 has a slit 24 formed through the top wall thereof (as oriented in FIG. 2) for passage of electrons emitted by heated filament 25 positioned outside chamber 22 and juxtaposed with slot 24 such that the electrons emitted from the filament are forced downward. Filament 25 is connected to a source of suitable electric power designated V which may be AC or DC The two side walls and bottom wall of ion source chamber 22 may be formed from the same nonmagnetic, electrically conductive material as the top wall, and all four such walls may comprise a single sheet of metal formed into the proper square or rectangular shape. A small diameter tubing 26 passes through the side wall of chamber 21 and the nearest sidewall of ion source chamber 22 and is connected at an input end (not shown) to a source of the sample gas being analyzed by my spectrometer. The front wall 27 of ion source chamber 22 is a grid of electrically conductive fine 'wires which is suitably connected to the adjacent two side walls and bottom and top walls of chamber 22 and therefore is at the same relatively high positive DC voltage V as such walls. Grid 27 may be a plurality of parallel, equally spaced wires disposed vertically, or horizontally, or both vertically and horizontally as illustrated in FIG. 2. The back wall of ion source chamber 22 may be fabricated of the same electrically conductive material as the other walls and directly connected thereto, or, as depicted in FIGS. 2 and 3, may be electrically insulated from the side and top and bottom walls of chamber 22 and connected to a source V of DC voltage which is slightly higher than the voltage impressed on the side, top, bottom and front grid wall of ion source chamber 22. The insulated and higher voltage back wall 28 of chamber 22 thereby forms an ion repeller plate for purposes to be described hereinafter.

As described above, 270 sector chamber 20 encloses the 90 sector chamber 21 completely on two sides thereof as illustrated in the FIG. 1 embodiment, or encloses the two sides partially as indicated in FIG. 2 wherein chamber 21 is of greater height than chamber 20. Since the purpose of chamber 21 is to contain the ion source and the ion collecting means at the output slit of my spectrometer, it is evident that the length dimensions of the two sides of chamber 211 oriented with the radii of chamber 11 may be equal to, greater or smaller than the radial dimension of chamber 20, a minimum radial dimension of chamber 21 being limited by the relative positions of ion source chamber 22 and the ion collecting means 29 contained in chamber 21.

The 270 sector nonmagnetic chamber 20 is comprised of a 90 sector portion 30 and a 180 sector por-. tion 31 as depicted in FIG. 2 and more clearly in FIG. 3. The sector portions 30 and 31 are the two stages of my spectrometer in which the ions undergo mass separation. The 90 sector portion 30 comprises the first stage mass separator and is provided with an ion entrance partition 23 and a partition 32 angularly displaced therefrom by 90 and being a common partition with the l sector portion 31. The geometric orientation of partition 23 is such that it is preferably along a radii of chamber 11 whereas partition 32 is along a diameter thereof. Formed within partition 23 is a second grid 33 which again may be comprised of parallel, equally spaced wires oriented in a vertical or horizontal or both vertical and horizontal positions as in the case of grid 27. The main distinction between grids 27 and 33 is that grid 33 is electrically isolated from grid 27 and is at zero voltage as are partitions 23, 32 and the remaining side, top and bottom walls of chamber 11. Grid 33 in general has an area dimension equal to that of grid 27. Partition 32 is provided with a first narrow slit 34 oriented vertically and extending along the major portion of the height of chamber 20. In general, the vertical length of slit 34 is approximately the same dimension as the vertical length (height) of grid 33 and 27. The sector portion 30 is therefore enclosed on all sides except for slit 34 and the interstices in grid 33. In like manner, ion source chamber 22 is enclosed on all sides. except for slit 24 and the interstices in grid 27, although repeller plate 28 may be slightly spaced from the side, top and bottom walls of chamber 22, as desired. The important factor is that the interior of the combination of chambers 20 and 21 (i.e., chamber 11) is totally enclosed such that the only means of introducing gas into the interior thereof is through tubing 26. A vacuum' pump (not shown) may be connected to the second stage 31 by means of tubing 36 passing through the side wall thereof in order to maintain the gas pressure throughout chamber 11 in the order of 10' torr, or less.

My ion source and the first stage of mass separation functions in the following manner. Electric power of DC or AC type is supplied to filament 25. Upon being energized, filament 25 emits electrons which are ac celerated in a downward direction by the electric potential existing between the low voltage on the filament and the relatively high positive voltage impressed on chamber 22. In the case of the pole faces l2, 13 covering a 360 span, the electrons are more strongly guided downward due to the magnetic field. The elec trons pass through slit 24 and collide with the gas molecules of the sample gas introduced intothe interior of chamber 22 through tubing 26 and thereby generate ions. The electric potential existing between grids 27 and 33 forms a relatively wide beam of the ions which is accelerated as a parallel entrance beam into the first stage mass separator 30. The width or crosssectional area of the entrance ion beam is approximately the area of grid 33. The particular ions which are to be monitored in the presence of other type ions generated within chamber 22 are focussed upon slit 34 by the interaction of the homogeneous magnetic field and grid 27-33 accelerating potential. The more heavy ions impinge on the partition 32 along the outer edge of chamber 11 and the lighter ions impinge on such partition closer to the center of the chamber relative to slit 34. The relative radial position of slit 34 in partition 32 is determined by the focal path of the ions in their passage through the two stages of mass separation. The relative radial position of vertical ion exit slit 35 at the output end of the second stage mass separator 31 is also determined by the focal path of the monitored ions. The monitored ions are thus focussed on slit 34 in the first stage and slit 35 in the second stage to obtain the two stages of mass separation. Since slit 35 is also located on the diameter oriented partition 32, but angularly displaced from slit 34 by 180, the second stage is seen to be a 180 analyzer or mass separator. Thus, my two stage mass spectrometer includes a 90 stage and a 180 stage. Since only a single magnet is employed in my spectrometer, the homogeneous magnetic field generated by the magnet causes the magnetic fields in the two stages to automatically track each other and thereby obtain a readily predetermined focal path for the monitored ions.

My spectrometer may thus be utilized with any readily ionizable gas by proper adjustment of the magnitudes of the magnetic field and grid 27-33 accelerating potential to obtain the focal paths through slits 34 and 35 for the particular ion being monitored. The ions focussed on exit slit 35 are collected in a suitable nonmagnetic, electrically conductive chamber 29 electrically insulated from partition 32. An electrical conductor 37 is connected to ion collector 29 and passes through a side wall of chamber 11 to provide the spectrometer output in microamperes as one example. The output current detected in conductor 37 is a known function of the number of ions focussed on exit slit 35, and thus the trace amount of a particular gas in a sample gaseous mixture can readily be determined.

As one example of the size of my spectrometer, chamber 20 may be of 3 -inch radius and 1 -inch height and chamber 21 is of the same radius and 2 -inch height. Grids 27 and 33 are each approximately 1 inch by 1 inch. Slits 32 and 35 are each of 0.02 -inch width and approximately 0.9 -inch height. The distance between grid 27 and repeller plate 28 is approximately three-eighths inch and filament 25 and slit 24 are threefourths inch long. Voltage V impressed on ion source chamber 22 is +l,000 V., voltage V, impressed on repeller plate 28 is +1 ,002V. and chamber 11 is maintained at ground potential.

From the foregoing description, it can be appreciated that my invention makes available a new twostage mass spectrometer and method of mass separation. Only a single magnet is employed thereby overcoming the large size alignment and synchronization (magnetic field tracking) problems associated with conventional twostage spectrometers using two separate magnets. The ion source generates a wide, parallel ion beam which is especially well adapted for magnetic focussing on the slit in the first stage. The two stages accomplish the desired effect of reducing the background intensity of unwanted ions by several orders of magnitude. The simple structure of the ion source and two-stage chamber provides an inexpensive spectrometer. My invention is defined by the following claims.

What l claim as new and desire to secure by Letters Patent of the United States is:

1. A two-stage, single magnet mass spectrometer comprising a single magnet having two opposing pole faces each spanning only a 270 sector and an air gap therebetween, v a 270 sector nonmagnetic first chamber juxtaposed in the magnet air gap, said first chamber comprismg a sector portion, and a sector portion,

said 90 sector portion comprising a first stage of the mass spectrometer and provided with an ion entrance partition and a partition common with said 180 sector portion, said common partition provided with a first narrow slit through which particular ions being monitored are focussed for passage from said 90 sector portion to said 180 sector portion,

said 180 sector portion comprising a second stage of the mass spectrometer and provided with an ion exit partition having a second narrow slit through which the particular ions are again focussed for passage from said 180 sector portion,

90 sector nonmagnetic second chamber juxtaposed with said first chamber in the 90 sector void associated with said 270 sector first chamber,

ion source means in communication with said ion entrance partition for generating a relatively wide, parallel beam of ions which is accelerated through said ion entrance partition for passage into said 90 sector portion, said ion source means comprising a nonmagnetic, electrically conductive third chamber positioned in said second chamber and provided with a narrow slit through a top wall thereof, said third chamber connected to a source of positive DC voltage,

a filament positioned outside said third chamber but within said second chamber and juxtaposed with the third chamber slit and connected across a source of electrical potential for generating a stream of electrons flowing through the third chamber narrow slit and to the interior of the third chamber,

means for introducing a sample gas at a low pressure to the interior of said third chamber whereby collisions of the electrons with the gas molecules generate ions,

a first grid formed of equally spaced parallel vertical and parallel horizontal electrically conductive fine wires forming at least a part of a first side wall of said third chamber, said first grid spaced from said ion entrance partition and substantially parallel thereto,

said ion entrance partition provided with a second grid formed of equally spaced parallel vertical and parallel horizontal electrically conductive fine wires maintained at zero voltage, the area dimension of said second grid being equal to that of said first grid, the electric potential between the first and second grids forming the relatively wide, parallel ion beam and causing acceleration thereof into said 90 sector portion of said first chamber,

an ion repeller plate forming at least a part of a second side wall of said third chamber and being electrically insulated therefrom and opposite the first side wall, said repeller plate connected to a source of positive DC voltage of magnitude slightly higher than the voltage applied to said third chamber,

vacuum pump means in communication with the 180 sector portion of said first chamber for maintaining a low gas pressure therein of approximately 10" torr, and

ion collection means in communication with the 

1. A two-stage, single magnet mass spectrometer comprising a single magnet having two opposing pole faces each spanning only a 270* sector and an air gap therebetween, a 270* sector nonmagnetic first chamber juxtaposed in the magnet air gap, said first chamber comprising a 90* sector portion, and a 180* sector portion, said 90* sector portion comprising a first stage of the mass spectrometer and provided with an ion entrance partition and a partition common with said 180* sector portion, said common partition provided with a first narrow slit through which particular ions being monitored are focussed for passage from said 90* sector portion to said 180* sector portion, said 180* sector poRtion comprising a second stage of the mass spectrometer and provided with an ion exit partition having a second narrow slit through which the particular ions are again focussed for passage from said 180* sector portion, a 90* sector nonmagnetic second chamber juxtaposed with said first chamber in the 90* sector void associated with said 270* sector first chamber, ion source means in communication with said ion entrance partition for generating a relatively wide, parallel beam of ions which is accelerated through said ion entrance partition for passage into said 90* sector portion, said ion source means comprising a nonmagnetic, electrically conductive third chamber positioned in said second chamber and provided with a narrow slit through a top wall thereof, said third chamber connected to a source of positive DC voltage, a filament positioned outside said third chamber but within said second chamber and juxtaposed with the third chamber slit and connected across a source of electrical potential for generating a stream of electrons flowing through the third chamber narrow slit and to the interior of the third chamber, means for introducing a sample gas at a low pressure to the interior of said third chamber whereby collisions of the electrons with the gas molecules generate ions, a first grid formed of equally spaced parallel vertical and parallel horizontal electrically conductive fine wires forming at least a part of a first side wall of said third chamber, said first grid spaced from said ion entrance partition and substantially parallel thereto, said ion entrance partition provided with a second grid formed of equally spaced parallel vertical and parallel horizontal electrically conductive fine wires maintained at zero voltage, the area dimension of said second grid being equal to that of said first grid, the electric potential between the first and second grids forming the relatively wide, parallel ion beam and causing acceleration thereof into said 90* sector portion of said first chamber, an ion repeller plate forming at least a part of a second side wall of said third chamber and being electrically insulated therefrom and opposite the first side wall, said repeller plate connected to a source of positive DC voltage of magnitude slightly higher than the voltage applied to said third chamber, vacuum pump means in communication with the 180* sector portion of said first chamber for maintaining a low gas pressure therein of approximately 10 5 torr, and ion collection means in communication with the second narrow slit for collecting the particular ions being monitored in the ion beam which pass through said first and second narrow slits and thereby undergo two stages of mass separation, the ion paths in said first and second stages being determined by the magnetic field generated by said single magnet thereby causing the magnetic fields associated with the two stages to automatically track each other. 